在Asp.net MVC4中访问ViewBag时出现NullReferenceException

Question

我正在尝试访问ViewBag我视图中的数据,如下所示:

<span class="small">@ViewBag.BreadCrumb</span>

我ViewBag从以下代码发送数据:

ViewBag.BreadCrumb = topic.Category.CatName + " / " + topic.Name;
ViewBag.TopicID = id;

这topic是实体.我使用以下命令返回View:

return View(topic);

但是,它总是给我一个例外:

Object reference not set to an instance of an object.

在另一个页面,它运作良好.有什么解决方案吗？

编辑::

行动守则如下:

Topic topic = db.Topics.FirstOrDefault(t => t.TopicID == id);
ViewBag.Topics = from t in db.Topics where (t.CatID == topic.CatID) select t;
ViewBag.TopicID = id;
ViewBag.BreadCrumb = topic.Category.CatName + " / " + topic.Name;
return View(topic);

编辑::

它显示以下异常:

An exception of type 'System.NullReferenceException' occurred in App_Web_ieekpj10.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

但是在立即窗口中,它显示输出:ViewBag.BreadCrumb"C Sharp Edit/Name-edited to CED"

编辑::堆栈跟踪

[NullReferenceException: Object reference not set to an instance of an object.]
ASP._Page_Views_Category_NewSection_cshtml.Execute() in e:\OutSourcingStuffs   \OnlineLibrary\src\OnlineLinkLibrary\OnlineLinkLibrary.Web\Views\Category\NewSection.cshtml:12
System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +271
System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +121
System.Web.WebPages.StartPage.RunPage() +63
System.Web.WebPages.StartPage.ExecutePageHierarchy() +100
System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext,   TextWriter writer, WebPageRenderingBase startPage) +177
System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance) +762
System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer) +382
 System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +431
System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +39
System.Web.Mvc.<>c__DisplayClass1a.<InvokeActionResultWithFilters>b__17() +74
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation) +388
System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +72
System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult) +303
System.Web.Mvc.Async.<>c__DisplayClass2a.<BeginInvokeAction>b__20() +155
System.Web.Mvc.Async.<>c__DisplayClass25.<BeginInvokeAction>b__22(IAsyncResult asyncResult) +184
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +136
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +56
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +66
System.Web.Mvc.<>c__DisplayClass1d.<BeginExecuteCore>b__18(IAsyncResult asyncResult) +40
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +47
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +68
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +47
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +65
  System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +39
System.Web.Mvc.<>c__DisplayClass8.<BeginProcessRequest>b__3(IAsyncResult asyncResult) +45
System.Web.Mvc.Async.<>c__DisplayClass4.<MakeVoidDelegate>b__3(IAsyncResult ar) +47
System.Web.Mvc.Async.WrappedAsyncResult`1.End() +151
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +59
System.Web.Mvc.Async.AsyncResultWrapper.End(IAsyncResult asyncResult, Object tag) +40
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +66
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +38
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9688704
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

编辑视图

@model OnlineLinkLibrary.Models.Section
@{
   ViewBag.Title = "New Section";
}

<div class="panel panel-default">
<div class="panel-heading ">
    <div class="row ">
        <div class="col-xs-6">
            <h4>
                New Section
                <span class="small">@ViewContext.Controller.ViewBag.BreadCrumb</span>
        </h4>
    </div>
    <div class="col-xs-6 ">
        <div class="pull-right"> 
          -- satic htmls are there
        </div>
    </div>
</div>
</div>

Answer 1

如果您 100% 确定ViewBag 不为 null，但 Visual Studio 始终将该行指向 null，

然后查看下一行代码的错误，编译器有时会感到困惑；）

这可能会帮助溺水的开发人员。

Answer 2

我建议检查主题是否为空。

Topic topic = db.Topics.Where(a=>a.TopicID == id).FirstOrDefault();
if(topic != null)
{
  ViewBag.Topics = from t in db.Topics where (t.CatID == topic.CatID) select t;
}